The invention belongs to the field of electrical switches and particularly to an isolation switch for a system supplying electrical power to a load such as one or more large three-phase induction motors.
An isolation switch is a switch intended for isolating an electric circuit from its source of electric power. It is intended to be operated only after the circuit has been opened by some other means, such as a main switch or circuit breaker, hence it it not designed to interrupt or establish current which would flow under normal operating conditions.
Bulk materials handling conveyors, and mining and loading machines, such as those used in underground coal mines, are examples of high horse-power, high voltage electrically powered equipment for which isolation switches are used to protect electricians servicing the electrical components. Further, electrical power equipment used at or near the working face in a coal mine must be "explosion-proof" to prevent electrical sparks or arcs from igniting any explosive mixture of air and methane or coal dust which may be present. Hence any electrical isolation switch intended for use on coal face equipment must be in an explosion-proof enclosure. Isolation switches previously available have not been easily or inexpensively adaptable for use within explosion-proof enclosures.
Coal mining technology continues to advance rapidly and the power requirements of mining machines continues to increase, probably doubling within the past decade. Motor sizes have tended to increase and utilization voltages have increased as a result. Isolation switches are becoming more and more desirable because of safety concerns, as a means of isolating utilization equipment from the main power supply. Machines are generally located several hundred feed from the main power center, the point at which power is distributed at utilization voltage. Locating the isolation switch within the same enclosure as the motor control gear allows the mine electrician to isolate equipment for repair or diagnosis without having to do so at the remotely located power center. An isolating switch is required by national standards to have means for readily connecting the load-side conductors to ground when disconnected from the source of supply. In addition, it must be possible to verify by visual observation that the switch is actually open and the load side is grounded.
The control equipment enclosure is generally located near the working coal face. Hence any isolation switch intended for use within the control gear enclosure itself must be suitable for installation in an explosion-proof enclosure.
Electrical motors and switches have rotor shafts and handle operating shafts extending from the inside to the outside of an otherwise sealed enclosure. Explosion-proof operation is achieved by journaling the shaft in a bore extending through a hub and providing sufficient axial length and minimum diametrical clearance that any flaming gas resulting from an explosion inside the enclosure will be quenches to a safe temperature by the time it exits through that clearance into the ambient atmosphere.
The clearance between the shaft and bore must not exceed a maximum deemed safe by the Mine Safety and Health Administration (MSHA). Control of that clearance is facilitated by the present invention by making the hub an integral part Of the switch assembly. By contrast, this critical clearance is more difficult to control with conventional isolation switch designs in which the switch is supported inside an enclosure in two planes. That is, it is fastened to one wall having the bore through which the operating shaft extends, and to another wall at right angles to it. Adjustment of the concentricity of the shaft in the bore is made by shifting the attachment mounting on the other wall.
Examples of such conventional isolation switches which when used in explosion-proof enclosures require adjustments in two planes are the Westinghouse Model #3DE1051-2LM Isolator and Kearney Catalog #504012-1 Isolator.